Waveguide to coaxial transmission line transition



June 16, 1959 E. s. LEWIS ETAL i 2,891,225:

wAvEGUIDE To coAxIAL TRANSMISSION LINE TRANSITION Filed oct. a. 1957 INVENTORS EDWIN S. LEWIS ERI: A- Tz-mwLEss NN E .NSN

United States Patent Orifice 2,891,225 Patented June 16, 1959 WAVEGUIDE To coAXIAL TRANSMISSION LINE TRANSITION Edwin S. Lewis, Merchantville, and Eric A. Thowless, Marlton, NJ., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application october s, 1957, serial No. 689,021 j 1 claim. (c1. ass-9s) This invention relates to wide-band microwave transmission systems and more particularly to a novel coupling arrangement for joining a rectangular waveguide l to a coaxial transmission line.

When a rectangular waveguide and a coaxial transmission line are coupled together to form a microwave transition, the center conductor of the coaxial line extends into a cavity within the waveguide. If the wavei I.1

guide is excited with radio frequency energy in the microwave region, this energy will cause an electric eld to be set up in the cavity. This field will in turn excite a TEM mode in the coaxial line. t

If the center conductor of the coaxial line is not acthe cavity is resonant to the trapped energy, a high voltf age standing wave ratio (VSWR) results, looking into the cavity from either the coaxial line or from the waveguide. The magnitude of the VSWR and the frequency band over which it exists depends on the -amount of excitation of the undesirable mode caused by the non-symmetrical i strtucture and the Q of the cavity. The VSWR is usually `at a maximum over a very narrow range of frequencies generally occurring about the midpoint of the frequency band which may be transmitted through the waveguide and coaxial line.

The undesirable mode can be minimized by using extreme care in manufacture to accurately, to a very small tolerance, center the center conductor in 'the cavitywithin the waveguide. This, however, results in higher manufacturing costs. The problem is particularly acute when a waveguide to coaxial transition is used in a rotary joint. In such a joint, some loosenness is needed for easy turning, and also rotation of the center conductor accentuates the non-symmetrical eiects.

It is an object of this invention to provide a waveguide to coaxial transition wherein such extreme care in manufacture is not required.

Another object of this invention is to provide a waveguide to coaxial transition in which the VSWR is minimized despite non-symmetry in the transition.

A third object of this invention is to provide a waveguide to coaxial transition which is economical to manufacture.

Another object of this invention is to provide a waveguide to coaxial transition particularly suitable for use in a rotary joint and in which rotation of the center conductor of the coaxial line will not accentuate non-symmetrical effects.

This invention accomplishes these objects by providing a means for absorbing or suppressing undesired modes in the cavity of a waveguide to coaxial transition. The transition comprises a coxial transmission line whose outer conductor is electrically connected to one wall of a waveguide while the center conductor extends within the waveguide. The center conductor is terminated in a conical conductive member tapering outward as it approaches the wall opposite from the conductors entrance into the waveguide. A plurality of small buttons of lossy material are located on the inner side of the waveguide wall adjacent to the base of the conductive cone. Any undesired modes caused by non-symmetry in the structure of the transition are substantially attenuated by the buttons of lossy material while the desired mode is virtually unaiected.

Other objects and many of the attendant advantages ofthis invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. 1 is a side view of a waveguide rotary joint broken away to show in cross section the waveguide to coaxial transition of this invention;

Fig. 2 is a cross-sectional view taken along the line 2-2 of Fig. l and a schematic illustration of an Iundesirable mode set up in the transition; and

Fig. 3 is similar to Fig. 2 and taken along the same line, but shows the proper TE mode set up in the transition.

The rotary joint shown in Fig. 1 is designed to couple together a rectangular waveguide 11 and coaxial transmission line 12. Various bearings and housing members are required to support the rotary joint. Since these are entirely of conventional design and form no `part of this invention details thereof have been omitted, and `the housing is generally represented in cross section by the box 13.

In coupling the waveguide 11 to the coaxial line 12 to form the transition of this invention, the outer conductor 14 of the coaxial line is physically and electrically joined to the top wall 15 of the waveguide through a conventional choke section 16. To facilitate broad band operation the outer conductor 14 is tapered outwardly adjacent to its junction with the top 'wall 15 of the waveguide 11. The inner conductor 17 of the coaxial line extends into the waveguide 11 to the opposite or bottom wall 18. Support means for the coaxial inner conductor is provided as shown; this comprises: a graphited-bronze bearing 19 in which the end of the inner conductor 17 is seated and which in turn. is seated in a metallic bushing support 20. In the form shown the side walls 24, Fig. 2, are spaced at least one-half wave length apart. A broadband matching member 21 of conductive material is provided which has a shaped surface projected upward from and electrically connected to the bottom Wall 18 of the waveguide. -It is this surface which provides the broadband matching function. The base portion of this matching member 21 forms a part of the supporting means for the inner conductor of the coaxial transmission line 12. Specifically illustrated is a conically shaped matching member 21 which projects inward through the bottom wall 18 of the waveguide 11 and surrounds the end of the inner conductor 17. The space between the matching member 21 and the inner conductor 17 and also between the matching member 21 and the bushing support 20 provides an RF choke for the inner conductor 17. As is common with rotary joints of this type the waveguide 11 is provided with a broadband matching iris 22.

When a rotary joint was constructed in the manner thus far described, it was found that satisfactory performance could not be obtained. RF energy transmitted by the waveguide 11 to the waveguide to coaxial transition caused improper modes to be set up in the transition which could not be propagated in the coaxial line 12 for transmission to the radar receiver. The same was true when attempting to transmit RF energy in the reverse direction from the transmitter through the rotary joint to the antenna. The improper or undesirable modes produced a VSWR as high as 1.8 which meant that approximately 8% of the energy injected into the transition in either direction wasfbein'g reflected. These nndesirable' modes 4are 'illustrated by the arrow's'23 inFig. 2. Though illustrated separately iny Fig. 2 it should be clearly understood vthat in 4fact they occur simultaenously with the desired mode which is illustrated by the arrows 25 inFi'g. `3.V I t was found that they were causediby eccentrieities in the coupling of the waveguide 11 andthe coaxial line 12. Rotation of the waveguide 11 from one position to another relative to lthe coaxial line Varied the ,strength of the'seundesirable modes so as to accentuate unsatisfactory' performance; 'Continued efforts to im-v prove performance'by the exerciseof greater skilll in manufacturing we'remet withl failure. Attention was next centered on devising means to attenuate the undesirable modes without simultaneously attenuatingthe desirlable TE mode. `Mode Suppressors were designedwith various geometrical configurations and were made from rn'any different materials. Materials 'such as resistance cards, powdered iron, and carbon impregnated substances were employed. .Attempts were made to attenuate the undesirable modes by aiiixing the various Suppressors to theside walls 24y aswell as the bottom wall 18 of the waveguide cavity. Every attempt resulted in failurethe desired TE mode was attenuated as well as the undesired modes-until' the mode Suppressors described below were employed.

vIn'Fig.v 3 the arrows 25 illustrate a desirable TE mod e which will propagate in the coaxial line 1'2. This result was nally attained by positioning two buttons 26 of lossy material, one on each side of the conductive merri` ber 21 on the bottom wall 18 of the waveguide 11. Excellent performance was obtained by employing buttons having a diameter of 1A inch and a height of 1/s inch. It was found that the VSKWR was reduced from the prevous 'figure of l1.8 to a ratio of 1.1. This meant that instead of l8% of the energy being reflected, the use of the buttons 26 so improved performance that only 1A of 1% was reflected at the resonant frequency. The size of the buttons employed in such a waveguide to coaxial transition will vary depending upon the dimensions of thev transition, theRF/frequency ofthe energy to be' transmitted therethrough, the Q of the cavity,l'and"the ma'gnitude o f the assynietry in the waveguide to coaxial line'Y coupling. In general such buttons should have a diameterlnotgreater'than one-tenth of a wave length and .',a eight not greater than one-twentieth lof a wavelength. 50

The buttons 2 6 may be Vsquare or of some configuration other than round without detracting from their function. One example of a lossy button material which was found satisfactory was carbon loaded Bakelite, known by the trade name of Synthne When buttons of this material and of the size mentioned above were employed in a waveguide coaxial transition, it was found that undesirable modes were more than adequately attenuated whilethe desired TE mode was virtually unaffected.

Obviously many'modicationsand variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

What is claimed is:

In a broadband microwave transmission system a waveguide rotary joint comprising in combination a rectangular waveguide, the end, side, top and bottom walls of one 'end of said waveguide enclosing a resonant cavity, saidside walls being spaced apart a distance at least equal to one-half of one wave length, an aperture in said top wall providing an entrance into vsaid cavity, a coaxial transmission line having an outer conductor coupled to said top wall and an inner conductor projecting into said cavity through said aperture, said outer conductor being uniformly tapered outward to mate with said aperture in said top wall, a graphited-bronze bearing into which theend of said inner conductor is seated within said cavity, a cylindrical metallic support for said bearing, a conically shaped conductivelmember vsurrounding said end of saidrinne'r conductor'and said vmetallic: support, the base of said member being axed to said bottom wall, a 'pair of mode Suppressors axed to said bottom `wal1 on' opposite sides of said conicalmember, each of said mode Suppressors being llocated substantially midway between said conical member and one of said`side walls, each said inode suppressor 4comprising carbon-loaded Bakelite material and having a diameter not in excess of one-tenth of fone' wave length and a height not in excess of one-twentieth of one wave length, said rectangular waveguide including a broadband matching iris positioned therein opposite 'said end wall, and said outer conductor including a rotary radio-frequency choke section.

References Cited in the file of this patent UNITED STATES PATENTS 2,473,443 Ragan June 14, 1.949 '2,762,986 Reed Sept. 11, 1.9.56

. FOREIGN PATENTS 524,063 Canada I v Apr. 17, 1956 

